screen: provide const main() overload

src/helpers.cpp

@@ -21,6 +21,7 @@
 #include <algorithm>
 #include <time.h>
 
+#include "enums.h"
 #include "helpers.h"
 #include "playlist.h"
 #include "statusbar.h"

src/screen.h

@@ -120,7 +120,9 @@ bool isVisible(BaseScreen *screen);
 template <typename WindowT> struct Screen : public BaseScreen
 {
 	typedef WindowT WindowType;
-	typedef typename std::add_lvalue_reference<WindowType>::type WindowReference;
+	typedef typename std::add_lvalue_reference<
+		WindowType
+	>::type WindowReference;
 	typedef typename std::add_lvalue_reference<
 		typename std::add_const<WindowType>::type
 	>::type ConstWindowReference;
@@ -201,6 +203,9 @@ public:
 	WindowReference main() {
 		return w;
 	}
+	ConstWindowReference main() const {
+		return w;
+	}
 	
 protected:
 	/// Template parameter that should indicate the main type